The present invention relates to self-powered radiation detectors which are typically utilized for in-core nuclear reactor radiation monitoring. The conventional self-powered radiation detector utilizes a central emitter wire, insulating means about the emitter wire, and a coaxially disposed collector sheath about the insulating means. The term "self-powered" relates to the fact that no drive potential is applied across the detector electrodes, but rather a signal current is generated as a function of the radiation response characteristic of the materials which are used for the emitter and collector electrodes. In general, the collector electrode is fabricated of a low neutron cross section, high temperature resistant, non-reactive material, such as the nickel-alloy Inconel, which is a trademarked material of the International Nickel Co. The emitter material is generally selected as the more radiation interactive material, and can be selected to be neutron responsive or gamma responsive based on the application and type of nuclear reactor.
The selection of emitter materials for such self-powered radiation detectors which are to be used in the core of a nuclear reactor must meet both mechanical and nuclear reaction considerations. Some of the properties which are desirable are good ductility, high melting temperature, desirable neutron cross section and/or gamma ray interaction probability. The fact that both mechanical and nuclear properties are to be satisfied tends to narrow the choice of materials to be used as the emitter in a self-powered detector. Some of the more widely used materials are rhodium and cobalt for neutron responsive detectors, and platinum for gamma ray responsive detectors. A metal which has a very desirable radiation response is lead, which has an almost pure gamma response, but has not found use for in-core application because of the low melting point of lead.
In a heavy nuclear reactor, such as the Canadian Candu reactor, the neutron flux and ratio of neutron to gamma radiation, is many times higher than for light water reactors of the pressurized water reactor type. This high neutron flux produces excessive burn-up of the neutron responsive material in self-powered detectors. The term burn-up refers to the atomic conversion of the material from a neutron interactive state to a relatively noninteractivity state. Currently, a widely used self-powered detector for such heavy water reactors utilizes a platinum emitter which is responsive to both neutron and gamma radiation. Such a platinum emitter self-powered detector produces a mixed response which is the sum of both the neutron interaction and gamma response of the material. Because of the high neutron flux and the burn-up of the neutron interactive component, the signal response will vary markedly with time. A more inherent problem is due to the difficulty of interpreting the generated signal due to the fact that it is a sum of a response to both neutron and gamma radiation.
It is therefore generally desirable to produce a self-powered detector which has a pure gamma response. A prior art attempt to produce a pure gamma response device utilized a nickel alloy steel such as Inconel as the emitter with platinum cladding about the Inconel emitter. This configuration lowers the neutron response from the platinum without significantly affecting the gamma response. This clad design still produces a mixed response although shifting it to be more gamma responsive than neutron responsive.